Duty Cycle stuck on 90% - 89 300SEL

[Durbo20vT]

I've been battling with finding the cause of this fault & have tried some tests found from searching BW without success. The car is an 89 M103, non-California car, so with my DMM plugged in ports 2&3, I should see 70%... but I do not.

My car reads 90.5% at all times from key on engine off, all the way thru running to 80* C. Flipping the poles in the diag port, i get 9.5% from KOEO thru running to full temp. The only time I get a fluctuation is upon throttle enrichment, where it moves to 60% then immediately back down to 90.5% upon liftoff. The car will start rough and die first attempt in the morning, but then starts to a normal idle of 750 during warm up phase & move to 1100-1200 rpm idle after reaching 80*C and stays there.

I've eliminated vac leaks & changed a ton of parts (list below). Hoping for a change, I unplugged the o2 sensor, deflected the air plate, tested voltage to IAC(11.7v)/potentiometer/EHA & all appeared to be well but did not see any change other than the CEL lighting up. No codes are stored when pulled with the code reader, just the 90.5% duty cycle (or 9.6%)...

The car is in what seems to be "limp home mode" & I'm looking to get out of it & adjust my duty cycle down to the 50% it should be at! I'm at the point where I've exhausted throwing parts at the problem, so what could I be overlooking which would correct this fault?


List of parts changed without success:

Brand new KAE OVPs (latest one 2 weeks old)
New Bosch 13925 o2 Sensor (last year)
New Potentiometer
New Fuel Pump & Filter
New Bosch Injectors & seals
Replaced IAC hoses
New Airflow meter boot
New throttle microswitch
Swapped throttle body+sensor with known working unit (new gasket)
Swapped EHA with a different unit
New Bosch Voltage Regulator
New Belt/Tensioner & associated parts
New Radiator (no impact expected)
Bosch Cap, Suppressor and rotor (1 year old)
Denso spark plugs (1 year old)
New 2-pin temp sensor (1 year old)
Swapped cracked hose from valve cover to breather tube on cyl head
New Bosch Coil
Replaced Fuel Pressure Regulator